Canon EOS 5D - Manual (Page 169)

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If a camera error occurs, “Err xx” will be displayed on the LCD panel.
Follow the instruction below to resolve the problem for respective error
code.
If the same error occurs often, something is probably wrong with the
camera. Jot down the “xx” error code and take your camera to the
nearest Canon Service Center.
If an error occurs after you take a picture, the camera might have
missed the shot. Press the <x> button to see if the image appears on
the LCD monitor.
Error Codes
Error Code Countermeasures
Err 01
Communications between the camera and lens is faulty.
Clean the lens contacts. (p.11)
Err 02
There is a problem with the CF card. Try any of the following:
Remove and re-insert the memory card. Formatting the CF
Card. Use another CF card instead.
Err 04
The memory card is full. Erase unnecessary images in the card
or replace the CF card.
Err 99
An error other than the above has occurred.
Press the shutter button halfway or remove and reinstall the
battery.
This error may occur if you use a non-Canon lens and the
camera or lens does not operate properly.